Conventional halftoning is an amplitude modulated process in which continuous shading is simulated by varying the size of equally-spaced printed dots. The resolution at which the dots are placed is called the line screen ruling. The dots are typically oriented at an angle with respect to the print substrate, and this angle is called the screen angle. In grayscale images, the screen angle is typically 45°, which makes the resulting halftone pattern less noticeable to the human eye. For color images, each of the primary colors (usually cyan, magenta, yellow, and black—CMYK) are printed at different screen angles in order to minimize color shifts due to misregistration during the printing process. The angles are carefully selected for each color in order to minimize the occurrence of interference patterns called Moiré (typical screen angles are C=15°, M=75°, Y=0°, K=45°).
Proofing is the process of generating a sample print, which represents the output that can be expected from a printing press. Proofing printers (“proofers”) are relatively inexpensive printers that strive to accurately represent the press output, including its color and quality. Color management for proofing devices is very important, because such devices generally use different color inks or donors than do the printing presses (proofer CMYK≠press CMYK). And even if a proofing device were to use the same color primaries as a particular press, such a device may not be usable with other presses. This is because not all of them use primary inks with the same hues, with American and European presses exhibiting significant differences in this area. Proofers may also need to match specialty or “spot” colors, which are essentially 5th or 6th colors used by the press for colors that the press's primary colors cannot reproduce well or at all.
In order to produce a color match on the proofer, the image data is typically color corrected before halftone processing. This process results in different data sets being halftone processed for the proofer and the press. Even if the same halftoning technique is used for both machines, the resulting halftone pattern for the two devices will be slightly different in order to make up for the color mismatch.
Some proofing devices focus on matching the color of the press, but appear not to be completely accepted because they use different halftoning techniques than the press. The feeling is that proofs from these devices cannot be used to predict the Moiré that can be expected on the press. Other proofing devices focus on matching the halftoning technique of the press as exactly as possible, but tend not to produce a good color match.